In quantum information systems, information is held in the “state” of a quantum system; typically this will be a two-level quantum system providing for a unit of quantum information called a quantum bit or “qubit”. Unlike classical digital states which are discrete, a qubit is not restricted to discrete states but can be in a superposition of two states at any given time.
Any two-level quantum system can be used for a qubit and several physical implementations have been realized including ones based on the polarization states of single photons, electron spin, nuclear spin, and the coherent state of light.
One way of transferring quantum information between two locations uses the technique known as ‘quantum teleportation’. This makes uses of two entangled qubits, known as a Bell pair, situated at respective ones of the locations; the term “entanglement” is also used in the present specification to refer to two entangled qubits. The creation of such a Bell pair is generally mediated by a light field sent over an optical channel (for example an optical waveguide such as optical fibre or silicon channels within a chip). Although this process is distance limited, where a respective qubit from two separate distributed Bell Pairs are co-located, it is possible to combine (or ‘merge’) the Bell pairs by a local quantum operation effected between the co-located qubits. This process, known as ‘entanglement swapping’, results in an entanglement between the two non co-located qubits of the Bell pairs while the co-located qubits cease to be entangled at all.
The device hosting the co-located qubits and which performs the local quantum operation to merge the Bell pairs is called a “quantum repeater”. The basic role of a quantum repeater is to create a respective Bell pair with each of two neighbouring spaced nodes and then to merge the Bell pairs. By chaining multiple quantum repeaters, an end-to-end entanglement can be created between end points separated by any distance thereby permitting the transfer of quantum information between arbitrarily-spaced end points.
Where a desired entanglement between two qubits is not directly created by a mediating light field interacting with both qubits in turn, the entanglement is effectively built up from multiple entanglements each involving a respective mediating light field; the qubit-to-qubit path taken by each such light field can be thought of as defining a build path segment for the desired entanglement, the aggregate of these build path segments defining an overall entanglement build path for the desired entanglement corresponding to the combined qubit-to-qubit paths taken by the mediating light fields. Thus, an entanglement formed between first and second spaced endpoint qubits that are coupled through a quantum repeater, will have a build path comprising a first segment between the first endpoint qubit and a first qubit of the quantum repeater, a second segment between the first qubit of the quantum repeater and a second qubit of the repeater, and a third segment between the second qubit of the quantum repeater and the second endpoint qubit; note that in this example the segments are not formed in order, the second segment being created last.
By controlling the path of a mediating light field, the endpoint qubits involved in an entanglement can be controlled. In terms of the entanglement build path, this corresponds to the selective routing of a build path segment whereby entanglement building is selectively routed to occur between desired endpoint qubits. Typically, routing of a light field is effected by an optical switch; again, in terms of entanglement build paths, such a switch can be considered to be one example of an entanglement build path switch.
Optical switches are complex and expensive and the present invention is concerned with providing an entanglement build path switch that reduces or avoids entirely the use of optical switches.